Polyamine depletion arrests growth of IEC-6 and Caco-2 cells by different mechanisms

The polyamines spermidine and spermine and their precursor, putrescine, are required for the growth and proliferation of eukaryotic cells. This study compares and contrasts growth arrest caused by polyamine depletion in the u ntransformed IEC-6 cell line with that in the p53-mutated colon cancer Caco -2 cell line. Cells were grown in the presence or absence of alpha -difluor omethylornithine (DFMO), a specific inhibitor of ornithine decarboxylase, t he first rate-limiting enzyme in the synthesis of polyamines. Depletion of polyamines inhibited the growth of both cell lines equally and over the sam e time frame. However, whereas IEC-6 cells were arrested in the G(1) phase of the cell cycle, there was no accumulation of Caco-2 cells in any particu lar phase. In IEC-6 cells, growth arrest was accompanied by elevated levels of p53 and p21(Waf1/Cip1) (p21). There were no changes in p53 levels in Ca co-2 cells. Levels of p21 increased in Caco-2 cells on day 2 without any ef fect on cell cycle progression. The amount of cyclin-dependent kinase (Cdk) 2 protein was unchanged by polyamine depletion in both cell lines. However, the activity of Cdk2 was significantly inhibited by DFMO in IEC-6 cells. T hese data suggest that in the untransformed IEC-6 cells the regulation of C dk2 activity and progression through the cell cycle are p53- and p21 depend ent. Growth arrest in the p53- mutated Caco-2 line after polyamine depletio n occurs by a different, yet unknown, mechanism.